This invention relates generally to a charging generator and, more particularly, to a charging generator which is used in a wide range of rotational speed of a generating unit.
Charging generators of this kind or, more specifically, charging generators generally used for motor vehicles have a circuit such as that shown in FIG. 1. That is, a generating unit 1 has an armature coil 101 and a field coil 102. A three-phase full-wave rectifier 2 constituted by six diodes are provided to rectify the alternating voltage output from the generating unit 1. A plus terminal of a battery 4 is connected to a plus output terminal of the three-phase full-wave rectifier 2 through a diode 902. One end of the field coil 102 is connected to the plus terminal of the battery 4. The other end of the field coil 102 is connected to the collector of a transistor 301 and is also connected to the plus terminal of the battery 4 through a diode 302. An output terminal of a voltage regulator 3 is connected to the base of the transistor 301. The voltage regulator 3 controls the field current flowing through the field coil 102 and, hence, the output from the generating unit 1. The output terminal of the three-phase full-wave rectifier 2 is also connected to the collector of another transistor 901. A control circuit 903 for controlling the short-circuit current of the generating unit 1 by switching the transistor 901 is connected to the base of the transistor 901.
In this charging generator, a field current supplied from the battery 4 flows through the field coil 102 and the transistor 301 while the generating unit 1 is being driven by an engine (not shown), and an alternating voltage is thereby induced across the armature coil 101. This voltage is rectified by the three-phase full-wave rectifier 2 and is applied to the battery 4 to charge the same. If at this time the terminal voltage of the battery 4 is higher than a predetermined level, the voltage regulator 3 switches off the transistor 301 to reduce the field current and, hence, the voltage induced in the generating unit 1. When the terminal voltage becomes lower than the predetermined level, the voltage regulator 3 switches on the transistor 301 to increase the field current and, hence, the induced voltage. The terminal voltage of the battery 4 is controlled in this manner to be maintained at the predetermined level. If the rotational speed of the engine is so low that the induced voltage cannot be increased to the level high enough to charge the battery 4 even if the field current is maximized, the control circuit 903 switches on the transistor 901 connected to the output terminal of the three-phase full-wave rectifier 2 to make the short-circuit current flow through the generating unit 1, and then switches off the transistor 901. When the transistor 901 is thereby switched off after being switched on, the current flowing through the armature coil 101 does not immediately become zero by the effect of the inductance of the armature coil 101, and a charging current therefore flows to the battery 4 through the diode 902.
In the thus-constructed conventional charging generator, the generating unit has only an output current proportional to the armature reaction, and a problem of difficulty in obtaining a sufficient output is therefore encountered. That is, during high speed rotation of the generating unit, the output from the generating unit is not increased or is reduced when the transistor 901 is switched to obtain the desired output.